Media queries is a 1 hour topic. This CSS Media Queries is a good tutorial to star with this topic. Well personally I don't think that designers today use media queries because most of them have shifted to bootstrap 4.

But what if you need to adjust the layout in some way? Surely you still need to understand media queries to be able to use bootstrap effectively? It's a tool, not a "wysiwyg" type substitute for learning CSS.

I have made complete websites with bootstrap only with no need of media queries at all.

What happens if there is a need to change a design detail for mobile/desktop? Perhaps a background image is used at larger screen sizes but not at smaller ones, or a simplified logo is used for mobile. These are common occurrences, I would have thought, and I can't see how it would be possible to achieve these changes without knowledge of media queries.

My point is that in order to use bootstrap (or any other CSS framework) effectively, you do need a good understanding of CSS, as well as the rules for the framework. There seems to be a common misapprehension that bootstrap is somehow a substitute for a sound basic knowledge of CSS, and that is not the case.

Media queries may seem daunting at first, but once you start playing with them, you realise they are actually quite straightforward. They should not take long to learn.

Bootstrap4 does not use css grid but uses flexbox to create its layout. It calls it a grid structure but has nothing to with css grid as such

Bootstrap is wholly dependent on media queries and has a whole range of them predefined by default in order to arrange the columns at smaller screen sizes. This is its biggest flaw as it takes no account of the design and why most professional coders avoid bootstrap.

Bootstrap can be useful as part of a large team or for a quick demo or for beginners but in reality it is just another layer on top of css that gets in the way. The things bootstrap does as far as laying out a page are concerned are far simpler using basic css.

I would advise that all beginners have a good understanding of CSS and media queries before learning bootstrap otherwise bootstrap becomes a crutch and not a tool.